The invention relates to a process and an apparatus for exactly and uniformly plating molten metal on a one-side of steel strip.
Industrial fields relative with automobile commercial world have recently desired such steel sheets treated with plating only on one-side thereof, i.e., so called one-side plated steel sheets, and there have been proposed many processes concerning this manufacturing technique. Typical examples are known as the exfoliating-agent coating process, the meniscus process, the one-side electrolizing process, the one-side grinding process or the supersonic plating process.
The exfoliating-agent coating process coats the exfoliating agent such as water glass on one side of the strip, plates on the other side on the dip-plating line, and brushes off the agent. But this process is involved with a problem requiring the coating of the exfoliating agent and the brushing off mechanism to reduce the working efficiency, besides the complicated processing steps.
The meniscus process is known by the Japanese Patent Laid Open No. 52-134826 (laid open to the public inspection in 1977) in which the plating is performed by contacting one side of the strip to the surface of the molten metal, utilizing the surface tension, and it has been said that this process could be reduced to the practice with more or less alternations to the existing facilities, however the rolls holding the strip is dirtied with the plating liquid at the same time of plating the one side, and the strip could not be travelled too fast.
The one-side electrolizing process performs the dip-plating of difference in the plated layer, and electrolytically exfoliates the thinner plated layer. This is rational but expensive in preparing the facilities.
The one-side grinding process plates on both sides in the usual process and mechanically removes the plated layer from one side with the brushing roll equipped on the in-line or the off-line. However, there are disadvantages in the life of the brushing roll or inferiority on the sheet surface after brushing.
The supersonic plating process is known by the Japanese Patent Laid Open No. 53-16327 (laid open to the public inspection in 1978) in which the strip is horizontally travelled 2 to 4 mm above the surface of the plating bath and the supersonic wave is ignited by a supersonic pendulum to its tip via a horn or bar connected to the supersonic pendulum which has almost the same surface as the still surface of the dip-plating bath, so that the tip is effected with the supersonic vibration to upheave the bath surface and contact the plate liquid to the one-side of the strip. However, the upheaval of the liquid surface by the supersonic wave is only around 2 to 4 mm and could not stand for changings of the shapes of the travelling strip or the vibration in the actual line.
In view of these circumstances, the attention has been nowadays paid to such a method which upheaves the surface of the plating bath by means of the pumps to contact it to the surface of the strip, and one of the examples is the Japanese Patent Laid Open No. 53-75124 (laid open to the public inspection in 1978) in which basically the bath surface is swollen in arch to which the travelling strip is contacted at its one side, however it is actually difficult to appropriately cause the plating bath to contact to the edges of the strip only by swelling the bath surface in the width of the strip, and as described in the same the protective gas is blown from the upper direction of the strip in order to avoid invasion of the plating bath to the other surface of the strip at the edges. Therefore, the cost for the facilities and the operation becomes expensive, accordingly.
According to this process, the wet length of the molten Zn and the strip manages the contacting time of Zn and the strip, and this contacting time plays an important role as reacting time at the dip-Zn plating. However, according to the prior art as above mentioned, the wet length obtained by one nozzle is only about 3 to 5 times of the width of the nozzle. For example, in a case of a nozzle having a slit of 10 mm in width, the wet length is only about 30 to 50 mm. That is, if realizing a reaction time corresponding to the reaction time of the continuous Zn plating line on the both sides, it is necessary to make the line speed extremely slow or install a plurality of the nozzles in a line direction. But the former reduces the production and the latter makes considerably large the amount of all exhausting Zn uneconomically, and further difficult problems are present in maintenance.
The present invention has been devised in view of such background of the prior art.
It is a primary object of the invention to perform a uniform and beautiful plating on one side of the strip without invasion of the molten metal to the other side not requiring the plating.
It is another object of the invention to lengthen the wet length in order to make the reaction time longer for plating the molten metal and heighten the supplying efficience of the molten metal.
It is a further object of the invention to follow changings of the width and length of the strip for providing the exact one-side plating.
Other objects and features of the invention will be apparent from description of preferred embodiments of the invention in referrence to the attaching drawings.